linux-kernel - Re: [PATCH RFC 2/2] vhost: support urgent descriptors

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Message-ID: <20140922065539.GA12057@redhat.com>
Date:	Mon, 22 Sep 2014 09:55:39 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Jason Wang <jasowang@...hat.com>
Cc:	Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH RFC 2/2] vhost: support urgent descriptors

On Mon, Sep 22, 2014 at 11:30:23AM +0800, Jason Wang wrote:
> On 09/20/2014 06:00 PM, Paolo Bonzini wrote:
> > Il 19/09/2014 09:10, Jason Wang ha scritto:
> >>>>  
> >>>> -	if (!vhost_has_feature(vq, VIRTIO_RING_F_EVENT_IDX)) {
> >>>> +	if (vq->urgent || !vhost_has_feature(vq, VIRTIO_RING_F_EVENT_IDX)) {
> >> So the urgent descriptor only work when event index was not enabled?
> >> This seems suboptimal, we may still want to benefit from event index
> >> even if urgent descriptor is used. Looks like we need return true here
> >> when vq->urgent is true?
> > Its ||, not &&.
> >
> > Without event index, all descriptors are treated as urgent.
> >
> > Paolo
> >
> 
> The problem is if vq->urgent is true, the patch checks
> VRING_AVAIL_F_NO_INTERRUPT bit. This bit were set unconditionally in
> virtqueue_enable_cb() regardless of event index feature and cleared
> unconditionally in virtqueue_disable_cb().

The reverse actually, right?

> So virtqueue_enable_cb() was
> used to not only publish a new event index but also enable the urgent
> descriptor. And virtqueue_disable_cb() disabled all interrupts including
> the urgent descriptor. Guest won't get urgent interrupts by just adding
> virtqueue_add_outbuf_urgent() since what it needs is to enable and
> disable interrupt for !urgent descriptor.

Right, we want a new API that advances event index but does not
set VRING_AVAIL_F_NO_INTERRUPT.
IMO still want to set VRING_AVAIL_F_NO_INTERRUPT when handling tx
interrupts, to avoid interrupt storms.

> Btw, not sure "urgent" is a suitable name, since interrupt is often slow
> in kvm guest. And in fact virtio-net will probably use "urgent"
> descriptor for those packets (e.g stream packets who can be delayed a
> little bit to batch more bytes from userspace) who was not urgent
> compared to other packets.
> 

Yes but we are asking for an interrupt before event index is reached
because something is waiting for the packet to be transmitted.
I couldn't come up with a better name.

-- 
MST
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